Speed up lunch/tapas/etc. shell utility functions.

1. Combined ~10 calls to the make build system to only one.
   We added a phony target "dump-many-vars" to the build system to dump
   "<var>=<value>" pairs. We then store the pairs as shell variables.
   With this cache get_build_var/get_abs_build_var can just return
   the shell variables instead of querying the build system.
2. Prune .git when we search for AndroidProduct.mks.

In internal source tree lunch time was reduced from ~15s to ~1.5s.

Bug: 27429759

(cherry picked from commit 08800fd9)

Change-Id: I862a0ec3c1aae97c552054dacec133e857042edf